The present invention relates to a method for increasing the oxygen concentration of an antimony-doped silicon single crystal to be used for a low-resistivity substrate for preparing n/n.sup.+ type silicon epitaxial wafers, especially of a silicon single crystal heavily doped with antimony which has a resistivity as low as 0.1 ohm centimeter or less.
In recent years, antimony-doped single-crystal silicon substrates have often been used as a n-type substrate for epitaxial wafers which are processed to semiconductor devices. However, the antimony-doped silicon single crystal, especially a low-resistivity silicon single crystal doped heavily with antimony, is too low in oxygen concentration to cause an intrinsic gettering effect. This has been a serious problem for the improvement of performance and yield in preparing semiconductor devices from epitaxial wafers making use of such a single-crystal substrate.
The reason an antimony-doped silicon single crystal has a lower oxygen concentration is considered to be as follows: The reaction between a quartz crucible and a silicon melt causes many volatile silicon oxides (SiOx) to be produced. The silicon oxides form a diffusion layer on the surface of the silicon melt. The silicon oxides are dissolved into the silicon melt in high concentration and collide with antimony atoms and antimony oxides. The collision makes the silicon oxides easily evaporate through the diffusion layer, when a reduced pressure is provided in the space of the furnace. Continuation of this condition causes the oxygen concentration of the silicon melt to be lowered, resulting in lowered oxygen concentration of the pulled single-crystal silicon rod.
The oxygen concentration of the antimony-doped silicon single crystal is preferably in the range of 16 to 22 ppma (parts per million atomic). It was impossible to obtain a silicon single crystal in this concentration range under conventional operating conditions. The pressure within conventional furnaces was at most 15 millibars (1500 Pascals). Consequently, it was impossible to obtain antimony-doped silicon single crystals of 16 ppma or more in oxygen concentration.
In order to prevent antimony from evaporating under reduced pressures inside the furnace, there is a known method wherein the pressure inside the furnace is maintained within the range of 40 to 100 torr, i.e., 53-133 millibars (5300-13300 Pa) (Japanese Patent Laid-Open Publication No. 62-292691). This pressure range surely prevents the evaporation of the antimony from the melt. However, if a silicon single crystal is pulled under this condition, the low segregation coefficient of antimony makes the material in the silicon melt enormously concentrated. The concentration of antimony in the single crystal becomes disadvantageously progressively higher toward the pulling direction of the rod. Further, crystal defects are prone to be generated under this condition with the result that crystallization cannot be successfully progressed during one entire pulling operation cycle.